The final few weeks of a marathon training
program can be hard
on the head... you start to have doubts about
your state of readiness, and maybe you're thinking of
going for one more long run. Don't do it! By now
you have achieved whatever fitness level you are
going to achieve.
Instead focus mentally on the race and try to
come up with a realistic time goal (I prefer a
range; ex: 3hrs 40 min to 3 hrs 50 min), and a
race strategy so you can deal with bad weather
or a difficult course.
Read more about the mental side of marathon
running here.